1. Field of the Invention:
The present invention relates to multifunctional catalysts and, more especially, to multifunctional catalysts useful in catalytic converters for the treatment of the exhaust gases from internal combustion engines.
Consistent with this invention, by the expression "multifunctional catalysts" are intended catalysts which promote oxidation, in particular, of the carbon monoxide and the hydrocarbons present in the internal combustion engine exhaust gases, and also the simultaneous reduction, in particular, of the nitrogen oxides present in such gases.
2. Description of the Prior art:
In gasoline fueled internal combustion engines, the composition of the exhaust gases may be adjusted to attain a stoichiometric equilibrium such that the catalytic reduction and oxidation of its various constituents produce water, carbon dioxide and nitrogen. The methods generally used for adjusting the composition of the exhaust gas in order to achieve this stoichiometry include, in particular, the continuous adjustment of the air/fuel ratio at the engine intake and/or the introduction of additional oxygen upstream of the catalyst. The composition of the exhaust gas can thus be varied over periods on the order of one second from a composition containing a relative excess of oxidizing compounds (referred to as a "lean" setting) to a composition containing an excess of reducing compounds (referred to as a "rich" setting), and vice versa.
In particular, the setting referred to as "lean" is such that the amounts of oxygen and nitrogen oxide present are greater than those required to oxidize the carbon monoxide, the hydrocarbons and the hydrogen which are present. Conversely, and in particular the setting referred to as "rich" is such that the amounts of carbon monoxide, hydrocarbons and hydrogen which are present are greater than those required to reduce the oxygen and nitrogen oxides which are present.
Catalysts for the pollution control treatment of such exhaust gases are, of course, known to this art.
Thus, European Pat. No. 27,069 assigned to the assignee hereof describes a catalyst incorporating a support based on a refractory oxide and an active phase consisting of cerium, iron, at least one metal selected from between platinum and palladium and at least one metal selected from between iridium and rhodium.
Published German Application No. 2,449,475 features multifunctional catalysts having cerium, iron, zirconium, rhodium and platinum as an active phase.
And published German Application No. 3,223,500 features multifunctional catalysts having rhodium, palladium, iron and cerium as an active phase.
However, such catalysts are found to have a stability over time which is insufficient to satisfy the present antipollution requirements.